The invention relates to metal smelting processes, such as ferrochromium smelting, which is generally carried out in a submerged-arc furnace. The operation of a submerged-arc furnace is based on conducting electric current between electrodes through the material to be smelted. The electric resistance of the material to be smelted generates thermal energy, so that the batch is heated, when a high electric current is conducted through the material to be smelted. The current is transferred to vertically positioned electrodes, which are located symmetrically in a triangle with respect to the furnace center point. When necessary, the positioning depth of the electrodes in the furnace is adjusted, because they are worn at the tips. The electrodes extend to the inside of the furnace via through holes provided in the lid.
Inside an arc furnace having a reducing atmosphere, there prevails a carbon monoxide atmosphere, which is mainly created from coke, semicoke or carbon contained in suitable coal which are fed in the furnace as reductants. Carbon monoxide gas is continuously removed from the furnace. Carbon monoxide gas contains solid particles.
When the burnt coke used as smelting furnace reductant is coked incompletely, and represents for example so-called “instant coked” material that is common in certain areas in the world, for instance in South Africa, the coke also contains tar, the volatile tar components of which are discharged from the furnace along with the carbon monoxide gas. The term ‘tar component’ here refers to a substance that is released from coke at a high temperature in an oxygen-free atmosphere. It is generally a mixture of short-chain hydrocarbons and long-chain hydrocarbons, aromatic hydrocarbons and sulfur.
In the prior art there is known, from the publication WO 2008/074912 A1, a method and arrangement where carbon monoxide gas containing solid particles is conducted from a smelting furnace first to a gas scrubber, which is generally a venturi scrubber, where a water jet is directed to the downwardly flowing carbon monoxide gas. Owing to the water jet, the solid matter contained in the gas is removed to water, and the cleaned gas is conducted out of the venturi scrubber through an outlet provided in the upper part of the scrubber. Then the flow rate of the carbon monoxide gas is increased by means of a blower. Thereafter the carbon monoxide gas is conducted to a particulate filter, by which the remaining solid particles are essentially removed from the carbon monoxide gas. The particulate filter illustrated in said WO publication includes a filtering chamber, inside which chamber there is arranged a filter element, through which carbon monoxide gas can be conducted, so that the solid particles remain on the surface of the filter element. Inside the chamber, there are provided cleaning nozzles, by which the inert gas blasting, for instance a nitrogen or carbon dioxide pulse, is directed to the filter element in order to remove the solid matter from the filter surface. Inside the chamber, underneath the filter element, there are arranged washing nozzles for feeding washing liquid in the chamber in order to slurry the solid matter removed from the filter to slurry. The slurry is collected in a discharge chamber, from which it is discharged.
The problem is that the tar component contained in gaseous state in the carbon monoxide gas is not removed, neither in the venturi scrubber nor in the particulate filter. Instead, as the temperature of the carbon monoxide gas after the venturi scrubber drops to about 30-50° C., the volatile tar component flowing therealong begins to stick as a viscous mass on the surfaces of the process equipment. It is condensated for instance on the rotor blades of the blower, and in the course of time, the rotor becomes imbalanced, in which case maintenance is needed. Tar is likewise condensated on the surfaces of the filter element in the particulate filter, thus blocking it, and is not removed by inert gas blasting.
The volatile tar component, which after said solid matter cleaning continues its procession along with the carbon monoxide gas, causes many further problems. Generally the carbon monoxide gas is recycled back to the process, where it can be used for example in the strand sintering of pellets for heating the sintering gas, in a preheating silo for preheating pellets etc., in which case the tar blocks burner nozzles, blasting holes and flow channels. The blockages caused by the tar result in a stoppage for performing the maintenance work, which brings forth high expenses.
Further, it is a particular drawback that even if the carbon monoxide gas obtained from smelting furnaces could be an excellent fuel to be used in power plants for the generation of electricity, the tar component contained therein renders it impossible to be used for example in a gas turbine power plant, because the tar would stick to the compressor blades of the gas turbine.